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1. Field of Invention
This invention relates to the apparatus and methods suitable for liquid carbon dioxide storage and process systems typically located at customer or user sites which supply liquid carbon dioxide (CO2) to devices which then utilize the liquid CO2 so as to provide various refrigeration effects. Such systems, while they may have many other beneficial uses, are especially useful as ground support/filling apparatus for trucks or rail cars expending liquid carbon dioxide for in-transit cooling, or for devices for food chilling or freezing or for making dry ice.
2. Description of Prior Art
Solid carbon dioxide (dry ice) has long been used as an expendable refrigerant for many cooling applications because of its ease of application, its non-toxity, its very large refrigeration effect when subliming, its direct change to the gas phase and its desirable low range of refrigeration temperatures. Dry ice, at atmospheric pressure, sublimes at minus 110xc2x0 F. and has a heat of sublimation (refrigeration) of 244 btu/lb. Initially, liquid CO2 typically was made at central manufacturing plants, converted to dry ice in the form of blocks and then transported to the customer or using sites, stored, then placed or mixed when and where cooling was desired. If CO2 vapor was desired for carbonation, the blocks were placed inside high pressure converters (about 1,000 psig) and allowed to warm to ambient temperature.
However, the inconvenience of handling dry ice and the attendant weight loss after purchase, but before use (which typically averaged 50%), caused the CO2 industry to change to liquid CO2 distribution and customer storage. The standard for the U.S. CO2 industry became about 0xc2x0 F. liquid CO2 with an equilibrium pressure of about 300 psig for distribution and customer storage. This temperature was selected as one that could be maintained readily by small single stage, air cooled freon type refrigeration units adjacent to an insulated customer storage vessel, with the coils for cooling the CO2 located in the ullage space of the customer storage vessel; and all so that the maximum allowable working pressure (MAWP) of the vessel was not exceeded. If vapor was desired for carbonation, etc., it was piped direct from the vessel""s ullage volume or for large users, a liquid CO2 vaporizer was utilized. If the CO2 was to be used for cooling, the liquid CO2 was piped directly from the customer storage vessel to the using device. Subsequently, about 10,000 such vessels with internal coils and attendant refrigeration units of various sizes have been installed within the United States. In addition, many variations of this arrangement have been produced. A fleet of liquid CO2 trucks are also in place to distribute liquid CO2, and liquid CO2 production plants typically produced liquid CO2 suitable in temperature and pressure to support this system. However, one lb. of liquid CO2 at these conditions converts to only about 0.47 lb. of dry ice, thus providing only a heat of sublimation (refrigeration) of about 115 btu per lb. of liquid CO2 used. During the conversion about 0.53 lb. of CO2 is released as vapor. Thus while the change from a dry ice distribution system to a liquid CO2 distribution system greatly reduced the losses of dry ice CO2 and eliminated the inconvenience of dry ice handling; the use of liquid CO2 for cooling applications imposed an undesirable CO2 loss. The steel chosen to fabricate the insulated storage vessel was chosen to be safe at low ambient temperatures and various insulations were used, including foam glass. More recently, vertical storage vessels with vacuum insulation are available, which typically do not contain internal coils, and which are suitable for temperatures as low as about minus 40xc2x0 F., and are replacing the older vessels.
It was well known that lower temperature liquid CO2 produced a higher percentage of dry ice/cooling when used, thus came a trend to production and distribution of lower temperature liquid CO2, so as to better support dry ice/cooling applications. Accordingly, in many geographic areas, a temperature of minus 20xc2x0 F. and 225 psig for liquid CO2 delivery became feasible. Virtually no changes in existing equipment was required to accommodate this lowered distribution temperature, and any vessel""s refrigeration unit, while less required, were left in place because of vacation and other low or non-use periods. However, principally because of metal safety concerns for the storage vessels, distribution equipment, etc., to further reduce the liquid CO2""s temperature at the production plant would require replacing much of the existing distribution equipment and customer storage vessels. At about minus 70xc2x0 F., CO2 begins to form a solid, and thus cannot be readily transported as a liquid, but a minus 65xc2x0 F. liquid produces about 0.57 lb. of dry ice, a conversion improvement of about 20%. It has been estimated that about 4,000 tons per day of liquid CO2 is used for cooling applications in the U.S., thus 800 tons per day could be saved if all could be cooled to minus 65xc2x0 F. before use. Accordingly, a number of refrigeration devices have been developed to cool liquid CO2 at the final use location for a wide variety of applications. Examples are: U.S. Pat. No. 4,888,955 issued December, 1989 to the present inventor, et al; U.S. Pat. Nos. 3,660,985 issued May, 1972, 3,672,181 issued June, 1972, 3,754,407 issued August, 1973, 4,100,759 issued July, 1978, 4,127,008 issued November, 1978, 4,211,085 issued July, 1980, 4,224,801 issued September, 1980, 4,693,737 issued September, 1987, 4,695,302 issued September, 1987, and 5,934,095 issued August, 1999, all to the present inventor.
While cooling liquid CO2 to low temperatures may seem to be a straightforward mechanical refrigeration problem; the highly unusual nature of CO2 (especially the triple point occurring at useful temperature and pressures) combined with the problems in moving a liquid that becomes a solid if allowed to de-pressurize (even momentarily) below the triple point pressure, combined to prevent a totally satisfactory solution. Some of the specific problems unique to CO2 and thus the industry include the facts that: (1) flowing liquid CO2 when de-pressurized even momentarily to about 60 psig (the triple point), almost instantly becomes a mixture of liquid and solid and only changes back to liquid with the relatively slow application of heat; and (2) in any subsequent flow, this mixture easily clogs lines, valves and use devices as additional solid/slush CO2 forms, and any subsequent pressure reduction will cause it to turn progressively solid. Accordingly, most prior art inventions did not move very cold liquid CO2 to a use point, without providing sub-cooling with a pump or by some type of gas pressurization.
A related problem is due to the nature of use of most expendable refrigerants, of which CO2 is member, whether used in liquid form or in solid form (dry ice). Expendable refrigerants typically are used precisely when the cooling is desired and in the exact amount needed, thus the use rate can vary greatly. Low use rates can be followed by high use rates, varying quickly from no use to very high use. Patents ""985, ""407, ""759, ""085, ""737, ""302, ""955, and ""095 all solved the problem of when very cold liquid CO2 is being used, by incorporating a storage function of previously cooled liquid CO2 for supply to CO2 using/dispensing devices along with the storage of warmer liquid CO2; thus storing the cold liquid CO2 in the sub-cooled condition. However, none were versatile enough to find wide use.
While sought for years and despite all these efforts, a sufficiently versatile solution to have wide applicability has evaded the CO2 industry.
The present invention provides methods and systems for safely receiving liquid CO2 at a range of temperature and pressures into either an existing or a new customer located storage vessel that, by temperature and pressure manipulation, is subsequently able to increase the liquid CO2""s refrigeration potential to the extent possible by cooling the liquid CO2 to between about minus 65xc2x0 F. and about minus 30xc2x0 F. prior to final use; and to maintain this liquid CO2 in the cooled and/or sub-cooled condition so it is available for ready flow to the use point without fear of dry ice blockages inadvertently occurring as it is being used. In one aspect, this hybrid system is able to incorporate use of the existing vessel refrigeration unit and standard events associated with distribution and use of liquid CO2 to simplify and minimize the size of the refrigeration equipment, without imposing the burden of discarding the existing equipment. It is modular, thus one or more of the system""s components (and in different sizes) can be installed, as best fits the individual users needs and equipment availability. Apparatus for maximizing the existing storage vessel""s (and its contents) potential refrigeration effect storage (thermal storage) for future utility is also included. In addition, in another aspect, the system is able to utilize the frequently largely unused, but already installed vessel refrigeration equipment. Accordingly, the modular system is able to be readily adapted to meet virtually all the different user""s sizes and pattern of liquid CO2 use requirements, but without the burden of custom designed and engineered systems or special customer station vessels. Thus a simple, add-on type modular and versatile system is provided that inter-reacts with most existing liquid CO2 production, distribution and customer storage and refrigeration equipment, so as to provide more efficient conversion of liquid CO2 to a colder or sub-cooled condition for those users who benefit from such additional cooling and reducing CO2 use by about 20%. Accordingly, one important aspect of the invention is incorporation in the process tank of a separate storage function for the high refrigeration potential liquid CO2, and the liquid CO2 stored in this separate process tank can be maintained in the sub-cooled condition, ready for instant use without fear of blockages. Another aspect is that the colder and/or sub-cooled liquid CO2 systems are able to recharge the storage simultaneously while the storage is being drawn upon by customer use. Still another aspect is that a storage vessel pressure control management system is included. One special advantage is that the size and of the storage vessel and the size of the sump or processing tank are independent of each other; and the size of both the deep cooling equipment and storage vessel refrigeration unit(s) are also independent. This allows selection of the receiving storage vessel""s size to include distribution economies; and selection of the processing tank""s size, and selection of both the deep cooling equipment and storage vessel""s refrigeration units"" sizes to include individual user CO2 needs/use patterns. This added equipment can be located near the receiving vessel or in circumstances where the CO2 use point is elsewhere, located so as to minimize the distance the chilled CO2 is piped to use. In still another aspect, one refrigeration unit can be provided which alternately either acts as a chiller for the storage vessel, or acts as a hybrid or modified binary cascade low temperature chiller for the process vessel, having a thermal storage/flywheel feature associated with the CO2 portion of the hybrid cycle. If desired, the chilled CO2 can be maintained in the sub-cooled condition without the use of a pump, so the pressure drop associated with flow can be accomplished without the CO2 flashing to vapor and interfering with the flow of liquid, so to provide predictable flow characteristics.